The present invention relates to a process for preparing an amorphous silicon semiconductor.
Since it was found by W. E. Spear et al that the conductivity of the amorphous silicon obtained by a plasma decomposition method of silane (SiH.sub.4) could be greatly altered by doping with phosphine (PH.sub.3) and diborane (B.sub.2 H.sub.6) and an amorphous silicon solar cell was fabricated on an experimental basis in 1976 by D. E. Carlson et al, the amorphous silicon solar cell has attracted much attention and studies with the aim of improving its efficiency have been numerous.
The Schottky barrier type, PIN type, MIS type and heterojunction type are known as structures of photovoltaic devices using an amorphous silicon thin film. It has been considered that the former three types have a bright future as high efficiency solar cells. For instance, D. E. Carlson et al obtained a conversion efficiency of 5.5% with a Schottky barrier solar cell in 1977. J. I. B. Willson et al obtained a conversion efficiency of 4.8% with an MIS type solar cell in 1978. Yoshihiro Hamakawa obtained a conversion efficiency of 4.5% with a PIN junction solar cell in 1978.
In the case of a PIN junction photovoltaic device, the characteristics thereof are greatly affected by the process for the preparation thereof. In many cases, the characteristics of amorphous silicon semiconductors are estimated by the photoconductivity, and the larger the photoconductivity of the thin film, the better. The photoconductivity of amorphous silicon semiconductors has a close relationship to hydrogen or fluorine included in amorphous silicon. A thin film of amorphous silicon containing hydrogen or fluorine shows a larger photoconductivity than amorphous silicon not containing them. The reason is considered to be that hydrogen or fluorine acts as a terminator for the dangling bonds. It is the present situation that the photovoltaic performance of a PIN junction photovoltaic device greatly varies depending on the preparation process, the preparation conditions, and so on, even if the content of hydrogen or fluorine in amorphous silicon to be formed into the respective thin film layers is the same and accordingly the photoconductivity of the undoped film is the same. Also, in any of the known processes, a desirable deposition rate of the film for providing a film having a good photoconductivity and a good photovoltaic performance is low, i.e. 1 to 2 angstroms/sec, and the known processes have the disadvantage that the productivity is low. The same obtains with the preparation of a photosensitive drum for use in plain paper copying, a charge coupled device or a thin film transistor. Accordingly, there is desired the development of a process for the preparation of an amorphous silicon thin film which can be practiced at a high film deposition rate and is excellent in junction characteristics.
It is an object of the present invention to provide a process for preparing an amorphous silicon semiconductor.
A further object of the invention is to provide a process for preparing an amorphous silicon semiconductor in which the deposition rate of a thin film is very high.
A still further object of the invention is to provide a process for preparing an amorphous silicon semiconductor which has an excellent junction characteristic.
Another object of the invention is to provide a process for preparing an amorphous silicon PIN junction solar cell having an excellent photovoltaic conversion efficiency.
It is a still further object to deposit an intrinsic amorphous silicon thin film on a P- or N-amorphous semiconductor to form a P-I or an N-I interface for a PIN homo- or heterojunction photovoltaic device in a magnetron type glow discharge decomposition chamber at a high film deposition rate with excellent junction characteristics and substantially without degrading the photoconductivity.
These and other objects of the present invention will become apparent from the description hereinafter.